1. Field of the Invention
The present invention generally relates to optical printing devices for exposing photosensitive recording media to form grayscale images. More particularly, the present invention relates to an optical printing device for selectively exposing a photosensitive recording medium based on image data, the optical printing device having a plurality of rows of multiple recording elements, including light-emitting elements such as light emitting diodes (LEDs) or electroluminescent (EL) elements, and switching elements such as liquid crystal shutter elements.
2. Description of the Related Art
A great number of devices for exposing photosensitive recording media to form grayscale images have been developed as devices that use instant films or colored paper, and are commercially available. FIG. 16 is a perspective view schematically showing a conventional optical print head mechanism disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-256928. In FIG. 16, white light emitted from a point-source halogen lamp 100 is separated into red, green, and blue light components by using a color liquid crystal shutter 101, which are then successively incident onto an end face of an acrylic rod 102 with a time lag. The acrylic rod 102 includes a reflective aluminum film deposited on the entire surface thereof except for a portion from which light emerges, and serves to convert the light entering from the end face into a line of light. Accordingly, linear red, green, and blue light is successively incident onto a black-and-white shutter array 103 with a time lag.
The black-and-white shutter array 103 includes three rows of pixels corresponding to red, green, and blue, respectively, through which only the light of a specified color may be transmitted. For example, when linear red light is emitted, the light is transmitted only through the row of red pixels, but is blocked by the other two rows of pixels. The linear red, green, and blue light converted by the black-and-white shutter array 103 is focused onto a photosensitive paper sheet 105 such as spectra-instant film manufactured by Polaroid Corporation by a SELFOC lens array 104 (SELFOC is a brand of condenser lens arrays). As the photosensitive paper sheet 105 is advanced relative to the black-and-white liquid crystal shutter array 103, the linear red, green, and blue light is sequentially incident onto the same location on the photosensitive paper sheet 105, thus forming a two-dimensional print image.
Conventionally, the photosensitive recording medium is exposed in this way and a grayscale image is thus formed. The two liquid crystal shutters, namely, the color liquid crystal shutter 101 and the black-and-white shutter array 103, are typically implemented by an STN (supertwisted nematic) liquid crystal, a ferroelectric liquid crystal, or the like capable of high-speed response in milliseconds by applying an AC voltage of approximately 10 kHz in order to achieve a short print time.
A large number of liquid crystal shutters are commercially available also as display devices. A device using a liquid crystal shutter includes two glass substrates, between which liquid crystal is inserted, and a spacer for preventing the upper and lower glass substrates from contacting each other. Typically, polarizers are disposed above the upper glass substrate and below the lower glass substrate so that their transmission axes are perpendicular to each other. The liquid crystal has a characteristic that an applied voltage (electric field) changes the orientation of molecules to become aligned along the electric field. For example, light is transmitted or otherwise reflected when a voltage is applied, and light is blocked when no voltage is applied. Furthermore, the intensity of the applied voltage is utilized for grayscale representation.
Driving methods of liquid crystals include a passive-matrix driving method and an active-matrix driving method. In the passive-matrix driving method, a stripe-like transparent electrode extending in the horizontal direction is formed on an upper glass substrate, and a stripe-like transparent electrode extending in the vertical direction is formed on a lower glass substrate, such that a voltage is applied to selective intersection points of the vertical and horizontal transparent electrodes to thereby control the transmission of light. In the active-matrix driving method, transistors are disposed at intersections of the vertical and horizontal electrodes to accumulate electric current at portions that form pixels.
In general, there are transmission- and reflection-mode liquid crystal displays. The transmission-mode liquid crystal display is of the type in which light from a backlight below the liquid crystal is transmitted through the liquid crystal to display an image. The reflection-mode liquid crystal display is of the type in which light is reflected by a reflector at the bottom below the liquid crystal to display an image.
The foregoing typical optical printing device has drawbacks in that high-speed and high-quality recording is not achieved at low cost. Such an optical printing device is designed so that rows of recording elements perform sequential exposures, causing color misalignment associated with time-lag exposures, resulting in degradation in image quality. Furthermore, data transfer to the print head is unsophisticated, leading to a low recording rate.